A shunt driver, also called a shunt regulator, is a voltage regulator often employed within a DC power supply. The shunt driver begins conducting at a specified voltage. It conducts as much current as required to hold its terminal voltage to a specified voltage by diverting excess current from a non-ideal power source to ground, often through a relatively low-value resistor to dissipate the excess energy.
A shunt driver can be used, for example, to drive a two-wire interface in industrial sensor applications. The two wires are employed for power supply of the shunt driver and a connected sensor, as well as, for communication. By this, the data provided by the sensor which has for example a digital output is transmitted. One wire serves as a reference potential, while the second wire serves for the supply and the output of the sensor.
In an off state of the output, only the power supply for the sensor, for example a supply current, is drawn from the two-wire interface. In an on state, the output is pulled down to a lower output voltage level by means of the shunt driver. Accordingly, the shunt driver is active during the on state of the output and not active during the off state.
Industrial sensors may be connected to their load and/or supply via very long cables which can be up to several 100 meters long. Such cable may have a large capacitance and inductance per unit length. Therefore, the impedance of the cable, as well as, the propagation delay of the cable may have a large range of variance. Also, the load itself can vary in a large range, can be purely resistive or have a large capacitive or inductive component. Furthermore, reflections occur due to a mismatch between a load and a cable impedance, i.e. the transmission line impedance and the long length of the cable. The reflected signal causes problems whenever the load impedance is much higher than the transmission line impedance of the cable. In these cases, the reflected wave will pull down the output below the minimum required supply voltage, which effect is called an undershoot. This can only be avoided if the voltage slope during a transition from the off state to the on state inclines not too steep. The shunt driver consequently has to fulfil the following requirements:                Prevent any undershoots of the output voltage in order to supply the sensor;        Offer a stable behavior for all kinds of loads connected to the output of the sensor.        
In general, shunt drivers are difficult to keep stable and at the same time realize a fast step response. Known active shunt drivers are based on an operational transconductance amplifier with a capacitive load in its control loop and two transistors coupled to the two-wire interface and the amplifier. These shunt drivers have a dominant pole at a very low frequency which slows down the transition between on and off state. Therefore, an objective technical problem consists in providing a shunt driver circuit and a corresponding method which achieve fast transition times without any undershoot in order to cope with long transmission lines.
The objective is achieved by the subject matter of the independent claims. Embodiments and developments are defined in the dependent claims.
The definitions as described above also apply to the description of the below embodiments unless stated otherwise.